Fibrinogen is a soluble plasma glycoprotein which is synthesized in the human body primarily by liver parenchymal cells. It is a dimeric molecule, consisting of two pairs of three polypeptide chains designated Aα, Bβ and γ, which are connected by disulfide bridges. The three polypeptide chains are encoded by three separate genes. The predominant form (HMW fib) harbors an Aα chain which is synthesized as a 625 amino acid precursor and is present in fibrinogen found in blood plasma as a 610 amino acids polypeptide chain, the Bβ chain contains 461 and the γ chain 411 amino acids. The three polypeptides are synthesized individually from 3 mRNAs. Assembly of the three component chains (Aα, Bβ and γ) into its final form as a six-chain dimer (Aα, Bβ, γ)2 occurs in the lumen of the endoplasmic reticulum (ER).
Fibrinogen circulates in blood at high concentrations (1-2 g/L) and demonstrates a high degree of heterogeneity. It is estimated that in each individual about one million different fibrinogen molecules circulate. Most of these variants, which account for just a small portion of the total fibrinogen (in most cases not more than a few percents), differ in function and structure.
Proteolysis of the carboxy-terminal part of the Aα chain results in three major circulating forms of fibrinogen having clearly different molecular weights. Fibrinogen is synthesized in the high-molecular weight form (HMW; molecular weight 340 kDa; the predominant form of Aα chains in the circulation contains 610 amino acids). The degradation of one of the Aα chains gives rise to the LMW form (MW=305 kDa); the LMW' form (270 kDa) is the variant where both Aα chains are partially degraded at the carboxy-terminus. In blood of healthy individuals, 50-70% of the fibrinogen is HMW, 20-50% is fibrinogen with one or two degraded Aα chains (de Maat and Verschuur (2005) Curr. Opin. Hematol. 12, 377). The HMW and LMW' variants show distinct differences in clotting time and fibrin polymer structure (Hasegawa N, Sasaki S. (1990) Thromb. Res. 57, 183).
Well-known variants which are the result of alternative splicing are the so-called gamma prime (γ′) variant and the α-ext Fib or Fib420 variant.
The α-ext Fib or Fib420 variant, which has a molecular weight of 420 kDa, accounts for 1-3% of the total circulating fibrinogen (de Maat and Verschuur (2005) Curr. Opin. Hematol. 12, 377). The extended α-ext Fib isoform is distinguished from the conventional α-chain of fibrinogen by the presence of an additional 236 residue C-terminus globular domain due to alternative splicing. Contradictary data on the function and characteristics of Fib420 is given in literature. Based on studies with plasma derived α-ext Fib, Applegate et al., Blood (2000) 95: 2297, concluded that the polymerization and cross-linking properties of α-ext Fib are not grossly different from plasma derived HMW Fib. They conclude that the additional C-domain has no effect on coagulation and they suggest that the function of the domain may be to support integrin-mediated cell adhesion. In EP 1 495 051 it is suggested that Fib420 might be less sensitive to degradation and could have an effect on clot structure. However, no substantiation is given and there is no suggestion as to whether the effect may be an enhancement or deterioration in clot structure or strength. Mosesson et al. (Biophys. Chem. 112, 209: 2004) have studied the ultrastructure of clots that are based on umbilical cord plasma-derived α-ext Fib. They reported that the fibers of α-ext Fib clots are thinner and more branched than those based on HMW fibrinogen, but that they have the same periodicity that characterizes all fibrin fibers. The authors suggest that the likely function for the extended α-chain in α-ext Fib is to provide sites for interaction with cellular integrins.